The present invention relates to the field of tool strings lowered downhole in wellbores. More particularly, the invention relates to a shock absorber for minimizing the impact of a tool string against other well equipment located downhole in a wellbore.
Shock absorbing devices have been developed to attenuate vibration and to reduce impact forces between tools, tool strings, and other equipment located downhole in a wellbore. Conventional shock absorbers use springs, elastomers, fluid dampening chambers, shear pins, and other devices to reduce impact forces. One specific application of a shock absorbing system requires protection for lubricator ball valves located downhole in a wellbore. Lubricator valves are located several thousand feet below the wellbore surface to seal the wellbore and to selectively permit the passage of tool strings such as perforating guns. Perforating gun strings can extend for hundreds of feet to facilitate the completion of horizontal and deviated wellbores. If a perforating gun string is accidentally released to fall into the wellbore, impact with the lubricator valve can severely damage the ball valve and delay future well operations.
One technique for reducing shock impacts uses shear pins or metal cutting devices to provide resistant forces. U.S. Pat. No. 4,679,669 to Kalb et al. (1987) disclosed a wireline shock absorber having a movable mandrel attached to carbide cutting elements. Shock was absorbed by the metal cutting elements, and the cross-sectional area of the cutting surface and the rake angle were altered to modify the desired resistance forces. In U.S. Pat. No. 5,566,772 to Coone et al. (1996), shear pins selectively permitted movement between inner and outer tubulars of a casing joint as the casing joint was landed in a wellbore. In U.S. Pat. No. 3,653,468 to Marshall (1972), a cutter bar sheared washer projections to reduce the shock against instruments dropped into a wellbore.
Shock absorbing systems are used in wellbore production strings such as pump sucker rods to reduce shock loads to the production equipment. As disclosed in U.S. Pat. No. 4,997,037 to Coston (1991), coiled springs protected moving components against shock loads. In U.S. Pat. No. 5,509,475 to Lewis (1996), a compression cushion and sleeve absorbed sucker rod stresses. In U.S. Pat. No. 3,923,105 to Lands (1975), elastomeric materials provided cushions between multiple guns in a perforating string.
Shock absorbing systems are also used in drilling strings to protect the drill bit and other drill string components. Such systems typically use springs and fluid dampening systems. U.S. Pat. No. 4,223,746 to Tanguy et al. (1980) disclosed a shock limiting apparatus having a hydraulic dampening device. When the deceleration reached a predetermined level, a device incrementally increased the frictional drag to dissipate kinetic energy. U.S. Pat. No. 3,947,008 to Mullins (1976) disclosed torsion tube springs for absorbing longitudinal shock loads. U.S. Pat. No. 4,246765 to Zabcik (1981) disclosed ring springs for providing shock absorption in a drilling string assembly, and U.S. Pat. No. 3,963,228 to Karle (1976) disclosed a coil spring for absorbing shock loads. U.S. Pat. No. 4,194,582 to Ostertag (1980) disclosed a double acting drill string shock absorber using springs for the return forces. U.S. Pat. No. 4,394,884 to Skipper (1983) disclosed a shock sub having inner and outer telescoping sleeves and a two chamber fluid damping system connected with a fluid carrying aperture. In U.S. Pat. No. 4,901,806 to Forrest (1990), axial forces in a drill string were dampened with dual pistons having corresponding fluid chambers. U.S. Pat. No. 4,055,338 to Dyer (1977) incorporated a lubricating passage between pressurized gas in one chamber and a liquid in another chamber to provide shock absorption. U.S. Pat. No. 4,133,516 to Jurgens (1979) disclosed a pressure equalizing piston for absorbing forces drilling operations. U.S. Pat. No. 4,552,230 disclosed a combination mechanical and hydraulic dampening system.
Other concepts have been proposed to provide axial load dampening or shock absorbing characteristics. U.S. Pat. No. 5,083,623 to Barrington (1992) disclosed telescoping concentric cylinders forming sealed chambers. Shock forces were cushioned by the metered movement of fluid from one chamber to another. Coil springs were also proposed to provide shock absorption. In U.S. Pat. No. 4,162,619 to Nixon et al. (1979), a drill string shock sub absorbed axial loads with a spring formed with a knitted wire fabric or rope compressed in to a compact mass. In U.S. Pat. No. 4,173,130 to Sutliff et al. (1979), a drill string shock absorbing sub was formed with polyurethane pellets contained in a lubricating liquid. In U.S. Pat. No. 5,133,419 to Barrington (1992), nitrogen gas or silicon oil was incorporated within the annular chambers of a metering piston to provide shock absorption. U.S. Pat. No. 4,817,710 to Edwards et al. (1989) disclosed resiliently mounted contact pads for providing a cushion in a tool string, and U.S. Pat. No. 4,693,317 to Edwards et al. (1987) disclosed compressible members for providing a shock absorbing system. In U.S. Pat. No. 5,183,113 to Leaney et al. (1993), a downhole decelerator used a plunger within the mud carrying casing. When the plunger nose impacted the landing plate, mud was pushed out of a chamber to decelerate the device.
The shock absorbing performance of conventional systems is limited by the mechanical characteristics of the devices, and the dynamics of fluid contained in the systems. A need exists for a reliable shock absorbing system which can fit within the narrow confines of a wellbore and which can absorb large amounts of kinetic energy.